A nozzle to be installed to a bottom of a molten metal vessel is adapted to discharge molten metal in an approximately vertical direction through an inner bore thereof, by using a hydrostatic head (hydrostatic height) of molten metal as motive energy. The inner bore of the nozzle is typically formed in a straight configuration where it extends straight and vertically, a configuration where a corner edge thereof on the side of an upper end of the nozzle is formed in an arc shape, or a taper configuration where it taperedly extends from the upper end to a lower end of the nozzle.
The nozzle includes a type having not only a function of simply discharging molten metal but also a function of controlling a discharge volume (discharge rate) and a discharge direction of the molten metal. For example, as for a continuous casting nozzle to be installed to a bottom of a molten steel vessel such as a tundish, an upper nozzle 1a has a flow-volume control device (e.g., a sliding nozzle (SN) device; see the reference numeral 12 in FIG. 4) on a lower side thereof, as shown in FIG. 4. The nozzle also includes an open type (open nozzle) 1b devoid of the flow-volume control device, as shown in FIG. 5.
It is known that, if turbulence occurs in a molten metal stream passing through the inner bore of the conventional nozzle, it will cause various problems, regardless of the presence or absence of the flow-volume control device. For example, the turbulence is liable to disturb flow-volume control in the nozzle having the flow-volume control device, or to cause scattering of a molten metal stream discharged from a lower end of the open nozzle to an open environment (see the reference numeral 15 in FIG. 5).
A factor causing turbulence in a molten metal stream passing through the inner bore includes an adhesion of molten metal-derived non-metal inclusions, etc. (hereinafter referred to simply as “inclusion adhesion”), onto the inner bore (see the reference numeral 14 in FIG. 4), and a change in configuration of the inner bore due to uneven wear of the inner bore.
In order to avoid the above phenomena, various measures have heretofore been attempted. For example, as measures for the inclusion adhesion, the following Patent Document 1 proposes to inject gas from a wall surface of an inner bore of a nozzle. Further, the following Patent Document 2 proposes to form a refractory layer resistant to the inclusion adhesion (adhesion-resistant refractory layer), on a wall surface of an inner bore of a nozzle. The technique of injecting gas from a wall surface of an inner bore of a nozzle and the technique of forming an adhesion-resistant refractory layer on a wall surface of an inner bore of a nozzle have been implemented in all nozzles to be communicated with a molten metal discharge opening, such as an upper nozzle, and a sliding nozzle device and an immersion nozzle to be provided beneath the upper nozzle, and it has been verified that the techniques have a certain level of inclusion adhesion-prevention effect. However, a position, a shape, a speed, etc., of the inclusion adhesion, often vary due to a difference in casting conditions between individual casting operations or a fluctuation in casting conditions in the same casting operation, so that it is difficult to fully prevent the occurrence of the inclusion adhesion. Moreover, it is necessary to provide a complicated structure for the gas injection, and/or the adhesion-resistant refractory layer, in each of a plurality of nozzle regions when a nozzle is formed in an integral structure (a single-piece nozzle extending in an upward-downward direction), or in each of a plurality of nozzles when they are formed in a divided structure (comprising an upper nozzle and an immersion nozzle aligned in an upward-downward direction). This leads to complexity in nozzle production process, and complexity in casting operation and management, which causes an increase in cost.
As measures for the scattering of molten metal discharged from the lower end of the open nozzle, the following Patent Document 3 proposes to form an inner bore to have a step portion with a specific shape, and the following Patent Document 4 proposes to form an inner bore to have a taper portion. Although each of the open nozzles disclosed in the Patent Documents 3, 4 has a certain level of effect in an initial stage of a casting operation under some specific casting conditions, it is not sufficient measures for the scattering, because there are problems that a difference in level of the effect occurs due to a difference or fluctuation in casting conditions, and the effect will become smaller along with an increase in elapsed time of the casting operation.